QEMU- 1CPU for guest while more cores used on host?

QEMU- 1CPU for guest while more cores used on host?

[robert.kucera]

Beginner's question: Got a host with KVM and two Opterons (2cpux16cores=32cores). Trying to virtualize some ancient intranet Redhat servers that cannot be upgraded due to some ancient scripts that would have to be rewritten (last upgrade was at least ten years ago). Of course amd-v+KVM is impossible to be used, but works well under built-in qemu. As the guest is only capable of using one CPU=one core, there is no point in giving this virtual machine more than one core(=cpu). However, as the only one core has got quite poor performance, my question is: Since Qemu is full software virtualization (without use of amd-v), is it somehow possible to have one cpu for guest while it would use more cores on the host? So that qemu process would be capable of using more than one core while guest would
  still see one cpu?

I don't know if this is even technically possible. If so, could you please give me a hint what keywords should I look for or even better some example?

Re: QEMU- 1CPU for guest while more cores used on host?

[Javier Guerra Giraldez]

1:  no. what you're asking can't be done.   if it was possible, every
chipmaker would implement it in silicon to create über-fast
single-processors on top of multicore chips.

2: why do you think "course amd-v+KVM is impossible to be used" ??  it
does work very well


-- 
Javier

Re: QEMU- 1CPU for guest while more cores used on host?

[Javier Guerra Giraldez]

On Wed, Jul 4, 2012 at 2:44 PM,  <robert.kucera@centrum.cz> wrote:
> Thank you very much for your explanation, it makes sense :-)
>
>>2: why do you think "course amd-v+KVM is impossible to be used" ??  it does work very well
> Not for me, it is some old RedHat version that fails on boot under KVM, but works well (but slow) under Qemu. It is even unable to use more cores, so the limit in this case is one-core per guest for me.

1: kvm does work, and very well on AMD chips.   "of course amd-v+KVM
is impossible to be used" is plain wrong

2: if an old OS doesn't work under kvm, but does under qemu, then you
can fiddle with kvm's cpu emulation flags.  try "kvm -cpu ?" to see
what's available.

-- 
Javier

Re: QEMU- 1CPU for guest while more cores used on host?

[Avi Kivity]

On 07/04/2012 10:59 PM, Javier Guerra Giraldez wrote:
> On Wed, Jul 4, 2012 at 2:44 PM,  <robert.kucera@centrum.cz> wrote:
>> Thank you very much for your explanation, it makes sense :-)
>>
>>>2: why do you think "course amd-v+KVM is impossible to be used" ??  it does work very well
>> Not for me, it is some old RedHat version that fails on boot under KVM, but works well (but slow) under Qemu. It is even unable to use more cores, so the limit in this case is one-core per guest for me.
> 
> 1: kvm does work, and very well on AMD chips.   "of course amd-v+KVM
> is impossible to be used" is plain wrong
> 
> 2: if an old OS doesn't work under kvm, but does under qemu, then you
> can fiddle with kvm's cpu emulation flags.  try "kvm -cpu ?" to see
> what's available.

What version is it?


-- 
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